The present invention relates to fuse/safe-and-arm/booster charge devices containing a flux compression generator (FCG) for producing a high current that drives a flying mass as a slapper, flying plate, or jet penetrator into explosives of a warhead or munition or to activate a hot wire so as to initiate and sustain a high level detonation in the insensitive high explosive or reactive materials contained therein.
At present, there are no methods to efficiently initiate or ignite relatively insensitive high explosives, propellants, or reactive materials, referred to as a whole as insensitive reactive materials. Conventional techniques use large powerful booster explosives to “boost” the final stage of an explosive train to obtain a complete and reliable reaction initiation in the main explosive fill when insensitive explosives are used for that fill. The need for such large systems stems from the very nature of the insensitive explosives that are designed by synthesis to guard against accidental initiation by exterior stimuli and render safe a munition or rocket motor. Such stimuli may include mechanical shock and impact, response to stray electrical power, and high temperature environments that are often encountered during storage, transportation and end use. The large boosters, having relatively sensitive explosives, present large vulnerable volumes to the stimuli and as such compromise the safety of the entire warhead and degrade from the intent of using insensitive materials for main munition fill. A compact device not having a large booster system but capable of reliably initiating insensitive material based on flux compression generator technology can vastly improve overall safety of the warhead system.
Further, when reactive materials are used as the fill or as structural components of munitions, there is a need to consume the materials at a very high rate. Since these materials have slow burn rates it may be necessary to use multiple ignition sites to vastly reduce total time for the entire combustion of the structure. Such electrical system could require a large capacitor bank that cannot easily be contained within the munition.
Various fusing techniques have been used in the past to initiate high explosives in munitions, such as described in Ordnance Explosive Train Designers' Handbook, NOLR 1111, 1952. A standard approach uses an explosive train involving an electric detonator that contains a small amount of highly sensitive explosive material (primary explosives) in a detonator, an intermediate sensitive explosive pellet, and a relatively large booster explosive having a contact interface with the main explosive fill (secondary explosives). Conventional secondary explosives, while less sensitive than primary explosives are considerably more sensitive than tertiary or insensitive explosives. In the explosive train, when the detonator is energized, it ignites the explosive within the detonator, which in turn initiates the explosive pellet, which in turn initiates a relatively large booster explosive, which in turn initiates the explosive fill. In this example, each explosive component in the train is sequentially less sensitive, allowing a build-up of energy as the train functions until finally the main explosive is initiated. In this system, the most sensitive explosives are smaller while the lesser sensitive explosives are larger. Thus, the small size of the sensitive material minimizes the vulnerability of a component to accidental function in response to environmental stress such as shock impact, stray electrical fields, and/or high temperature environments that can set off explosive materials.
Reactive materials can be initiated using the hot wire technique and in some cases shock from detonated explosives. Since use of reactive materials, and in particular difficult to ignite metal-metal oxide reactions, is now being contemplated, it likely will be necessary to supply large amounts of energy for reliable initiation of these materials, particularly when multiple point initiation is desired.
Generally, in application, a munition is designed to detonate upon a proper command that starts the sequence associated with the explosive train. For example, if the munition needs to be detonated upon impact with a given target, then the munition will be activated when an associated frontal switch is closed upon impact with that target, which in turn discharges current from an on-board capacitor or battery carried in the munition into the detonator to begin the sequence.
When using an explosive train, a serious concern relates to use of a detonator and its activation switch that are most vulnerable to accidental function when subjected to unwanted external stimuli. For example, if the munition were to be dropped accidentally during transport, then the switch could close and result in an undesired munition detonation. Further, even if the switch did not close, the detonator might see sufficiently high stress from the impact, causing its sensitive material to react. If the detonator functions, on purpose or not, the entire train is activated with a resulting detonation of the main charge. Consequently standard fusing uses an “out-of-line” mechanical technique wherein the detonator itself or the combination of the detonator and explosive pellet are place out of line relative to the rest of the explosive train and considered in the “safe” position. The detonator is brought “in line” only under very restrictive conditions related to some other aspect of the weapon system. If the munition were a projectile fired from a rifled barrel, then known levels of setback and rotational acceleration can be used to allow the detonator to be aligned with the rest of the explosive train after some desired time delay. Once aligned, the fuse is considered to be in the “armed” position. The time delay assures that the munition is not armed until the munition has traveled a safe distance from the gunner.
A more recent example of initiation is use of a slapper detonator to initiate munition fills directly with fewer elements in the explosive train (reference: www.teledynerisi.com RP-95 EFI Detonator Data Sheet, Excelitas Technologies, incorporated herein by reference.). The slapper detonator includes a secondary explosive element Hexanitrostilbene (HNS) that in turn ignites the secondary explosives of the main fill. This system uses an exploding wire technique powered by electrical energy and as such propels a Mylar or PTE film or metallic foil across a small gap at very high velocity to impact the fill. The high velocity impact generates sufficiently high pressure to initiate the secondary explosive element. Although the slapper detonator is relatively small (0.5 inch diameter), it requires a high voltage electrical source such as a highly charged capacitor. When used as an initiator in a munition, a fuse switch and out of line techniques can be used to provide a safe system.
With the advancement of microcircuits, a development of superior fusing based on “in-line” systems known as Electronic Safe Arm and Fire (ESAF) has been realized as state of the art technology (reference: www.excelitas.com Electronic Safe, Arm and Fire Devices and Modules, incorporated herein by reference.). In these systems, when a slapper detonator is used for example, the slapper detonator is in direct contact with the main explosive fill while the electronics provide the “safe and arm” functions. Such in-line systems meet reliability and safety requirements as set forth in Military Standards for fusing (reference: MIL-STD-1316E, Fuze Design, Safety Criteria for DOD, 10 Jul. 1998, incorporated herein by reference).
Tertiary insensitive high explosives require a substantial increase in energy over traditional secondary explosives to initiate and sustain detonation. Criteria for initiation relates to the “failure diameter” associated each explosive type. Conventional explosives typically have failure diameters ranging from sub-millimeters for primary explosives to one-half centimeter for secondary explosives like Comp B, while those for insensitive materials like TATB derivatives, Baratol, and Destex can range from 2.0 centimeters or more (reference: T. R. Gibbs and A. Popolato, Eds., LASL EXPLOSIVE PROPERTY DATA, University of California Press, Berkley, C A, 1980, incorporated herein by reference). The notion relates to long cylinders of explosives, wherein several of varied radii are detonated or attempted to be detonated. Below a certain diameter (failure diameter) the detonation fails to propagate. It is easily seen based on the previously mentioned failure diameters that an order of magnitude more energetic initiation schemes must be developed to reliably initiate insensitive munitions. These required levels of energy reduce the probability significantly that environmental stress or external stimuli can produce an undesired event in munitions that use insensitive reactive materials. Thus, use of insensitive munitions and explosive fills in munitions is an effective strategy to render weapons that are far safer than before.
Given that the energy density is rather uniform over the explosive surface related to the diameter, it can be noted that the energy required to sustain detonation increases exponentially with failure diameter. Thus, insensitive high explosives are difficult to initiate and require significantly larger booster explosives for initiation. A slapper detonator system using the secondary explosives HNS as booster explosives, for example, would need to be significantly larger. Then, the secondary booster of the slapper detonator becomes significantly more vulnerable due to its size. Having such detonator to initiate the insensitive explosive fill, or such detonators distributed throughout the main explosive fill as a multipoint initiation scheme introduces increased vulnerability for the entire munition and significantly reduces the benefit of using insensitive tertiary explosive fills.
The vulnerability of the slapper detonator booster could be reduced through use of insensitive explosives as its booster material. However, the slapper detonator's Mylar or metallic foil projectile would then need to be more massive, propelled to higher velocity, and more energetic. As such, the energy required for the projectile increases directly with mass and with the square of velocity. Consequently, for adequate function, a reduced vulnerability slapper detonator system using a booster of insensitive explosives would require a significantly greater energy source and therefore a much larger capacitor.
Another class of insensitive materials that are of interest is reactive materials that can have energy outputs as high a 4 to 5 times TNT explosives. These include both combustion types that produce gaseous products such as Aluminum/Teflon and heat producing types such as the solid-state reaction of metal/metal oxides like the Fe/A10 “thermite.” Some of these materials can have quality structural properties and are finding use as replacements for steel used as munition casings (Reference: J. Goldwasser, DARPA DSO, www.darpa.mil/Our_Work/DSO/Programs Reactive_Material_Structures_(RMS).aspx, April 2015, incorporated herein by reference). As such, the casing also can react, producing greater munition output in terms of energetic fragments and/or increased blast, for example. Another option is to replace both explosives and casings with a solid billet of reactive material to obtain significantly greater munition output.
A conventional means used to initiate reactive materials is a “hot wire technique” wherein an embedded resistive element (Nichrome wire) is Joule heated using electrical current above the reaction temperature of the reactive material. Typically, a current density of 1000 Joules per cubic centimeter is required for initiation (reference, T. P. Weihs, Johns Hopkins University, Woodhead Publishing Limited, Chapter 5, p. 160, 2014, incorporated herein by reference), which could imply some 0.5 Ampere for 2 seconds. A drawback of using reactive materials in munitions is that the reaction propagation rate is low, typically being from 1 to 100 m/s whereas explosives have rates on the order of 6000 to 9000 m/s. Although the energy release is high, the low rate precludes use as a high peak pressure mechanism to overcome stress levels required to damage targets.
The present invention having large current output can significantly reduce the reaction time by initiating the reactive materials at many sites within the structure or billet. For example, if the reaction rate were 10 m/s, then reaction time to consume a 1-meter long column of reactive material would be 100 milliseconds. When 100 equally spaced initiators are used, then each reaction only needs to propagate 0.005 meter to consume the entire length in 0.5 millisecond. Such multipoint initiation reduces time for total consumption and produces a far shorter blast pulse with a correspondingly higher peak pressure in the blast field. To accomplish this task conventionally, with energy stored in a reasonably sized capacitor, would require highly sensitive incendiary material to be spread throughout the bulk of the structure or billet, greatly increasing the vulnerability of the munition. The present invention in compact form can supply the required electrical energy to initiate the reactions at single or multiple initiation sites.
Since the energy required is beyond that provided by conventional initiation techniques, devices based on traditional techniques have not been able to reliably initiate insensitive high explosives or reactive materials with a reasonable size device. To provide sufficient energy, the initiation devices would need to be scaled to much larger size (large booster) for an explosive train or would need to contain large electrical storage devices (capacitors) to power the slapper detonator or hot wires. A shaped charge of large size could be used to provide a sufficiently energetic jet but because of the large explosive mass, it would have vulnerabilities similar to large boosters. The slapper detonator contains secondary explosives that have inherent vulnerabilities. Use of these at multiple initiation sites would distribute such secondary explosives, being more sensitive than insensitive reactive materials, throughout the insensitive munition fill. Explosive trains or boosters located at each one of the multiple sites would use up too much volume and increase the risk that accidental initiation or ignition could take place. Further, once the shaped charge or explosive train is initiated accidentally, the jet or booster would initiate the main insensitive explosives without a means to prevent its function.
Flux compression generators (FCGs) are already known in the art. An example thereof is disclosed in U.S. Pat. No. 4,370,576, issued to J. S. Foster, Jr., on Jan. 25, 1983, and the entirety of which is incorporated herein by reference. Further, FCG coupled with an electrical load to form jets and penetrators has been described in U.S. patent application Ser. No. 13/949,849, entitled Explosive Device Utilizing Flux Compression Generator, Grace et al, filed on 24 Jul. 2013, and the entirety of which is incorporated herein by reference.
A flux compression generator is a device that converts explosive energy directly to electrical energy. Although various geometries have been explored (reference: C. M. Fowler and L. L. Altgilbers, “Magnetic Flux Compression Generators: a Tutorial and Survey,” Journal of Electromagnetic Phenomenon, 3 (11), 2003, pp. 305-357, incorporated herein by reference.), a cylindrical geometry has attractive features for warhead and munition applications since these devices generally also have a cylindrical geometry. Of these, two types have been advanced and are known as “coaxial” and “helical” generators, respectively (reference: Fowler and Altgilbers, supra). However, the present invention can operate using any FCG geometry.
An FCG consists of a cavity having axially spaced input and output ends and enclosed by electrically conductive material such as metal, and a means, such as explosives, to collapse the cavity to a minimum volume. The cavity is created using two concentric metallic shells, the inner shell being referred to as an “armature” since it will move outward, and an outer stationary shell known as a “stator.” The inner shell is loaded with explosives, and held in position relative to the stator using metallic caps placed on each end. The space between the armature, stator, and end caps defines the volume associated with the cavity. Typically, the ratio of the radii of the two shells is slightly less than two, while the length to diameter ratio of the entire device is usually greater than one. Thus, a typical FCG has considerable cavity volume. The explosive may be initiated at the input end, while the other end is referred to as the output end. The input end cap inner radius is slightly smaller than that of the armature so that a small gap exists initially to feed current in and out of the two shells from an external source. A small radial gap also exists between the stator and the output end cap so that current can be directed outward into an external electrical load.
In operation, a relatively small amount of “seed” current from an external source is injected into the armature input end and returned from the input end of the stator. Thus, the flow of current is directed along the length of the armature, through the output end cap, continues into the external load circuit, and returns back through the stator and the input end cap to the external source. When a solid metallic stator is used (coaxial generator), the current flow establishes an azimuthally oriented “seed” magnetic field within the cavity. When the stator consists of helical windings (helical generator), current flow in the turns establishes a longitudinal “seed” magnetic field. Beyond that difference, the two types of FCGs operate in similar fashion.
After the “seed” current, and therefore the “seed” magnetic field has been established, the explosive is initiated at the input end. Thus, expansion of the detonated explosive causes the armature to begin to expand first at the input end and progressively down the armature length as the detonation wave travels toward the output end. The first motion of the armature is to cross the gap at the input end cap, make contact with the end cap, and cut the external seed current source out of the circuit so that the circuit now consists of the armature, the output end cap, the load, the stator, and the input end cap that is now in contact with the armature. Thus, the current and magnetic field are trapped within a closed volume represented by the FCG cavity together with the cavity of the electrical load. The armature continues to expand radially, reducing the FCG cavity volume to near zero when the output end of the armature collides with the output end of the stator.
During FCG function, the magnetic field, its associated pressure, and corresponding current are intensified as the cavity is collapsed. As the collapse process continues, the explosive driving the armature does work against the magnetic pressure and thereby converts its chemical energy released upon detonation to electrical energy. A nominal 40 mm diameter device with L/D (Length over Diameter)=1.5 FCG, using 3 thousand Amperes of seed current can produce 1.5 million Amperes of peak current delivered in about 10 microseconds. Thus, 1 kJ of electrical energy can be generated using an FCG having a volume of 100 cm3. Using state-of-the-art capacitors having charge densities of 2.5 Jules/cm3 would require a capacitor volume of 400 cm3 to match the output of a FCG. Thus, there is a great advantage of using an FCG to create energy to initiate insensitive explosives using electrical driven impactors or hot wires as compared to using a storage capacitor for that purpose.
One example of a coaxial generator that can be employed in devices according to the invention is disclosed in: J. H. Goforth, et al, “The Ranchero Explosive Pulsed Power System,” 11th IEEE International Pulsed Power Conference, Hyatt Regency, Baltimore Md., Jun. 29-Jul. 2, 1997. An example of a helical generator that can be employed in devices according to the invention is disclosed in: A. Neuber et al, “Compact High Power Microwave Generation,” Proceedings of the Army Science Conference (26th), Orlando, Fla., 1-4 Dec. 2008. The disclosures of these publications are incorporated herein by reference.
To a first order, the peak FCG output current results from the starting inductances of both cavities relative to the final inductance of the system after magnetic compression. When the FCG cavity is completely collapsed, current gain is the ratio of the initial cavity inductance (FCG plus the load) to the final inductance represented by the load. Consequently for a typical FCG system having load inductance of 2 nanoHenries and FCG cavity of 4000 nanoHeneries the current gain is about 2000.
An advantage of the helical generator with its wire wound stator is that a much higher initial inductance can be obtained per unit length, but at the expense of added complexity. In contrast, the coaxial generator has a simpler construction, but with a considerably lower initial inductance. For well-designed generators of similar length, typical current gains are 10 to 12 for the coaxial types, and 2000 or more for a helical wound generators. Often, coaxial generators are used with much higher seed current to get high output current since premature electrical breakdown between wires and wire melting are not issues.
Work with explosively driven flux compression in the United States dates back to C. M. Fowler's work published in 1960: C. M. Fowler, W. B. Garn, and R. S. Caird, “Production of Very High Magnetic Fields by Implosion,” Journal of Applied Physics, 31(3), 1960, pp. 588-594, incorporated herein by reference.
Since then, both coaxial and helical generators have been designed, built, and tested. The most notable groups examining helically wound generators include Los Alamos National Laboratory in Los Alamos, N. Mex., as disclosed in: C. M. Fowler and L. L. Altgilbers, “Magnetic Flux Compression Generators: a Tutorial and Survey,” Journal of Electromagnetic Phenomenon, 3(11), 2003, pp. 305-357, the Kurchatov Institute of Atomic Energy in Moscow, S. Kassel, “Pulsed-Power Research and Development in the USSR,” R-2212-ARPA, May 1978, and Texas Tech University in Lubbock, Tex., A. Neuber, et al, supra, all, incorporated herein by reference.
Notable patents pertaining to explosively driven flux compression devices with helically wound generators include U.S. Pat. No. 4,370,576, J. S. Foster and J. R Wilson, U.S. Pat. No. 3,356,869, J. L. Hilton and M. J. Morley, all incorporated herein by reference.
U.S. Pat. No. 4,370,576 details the operation of helically wound flux compression generators. J. L. Hilton et al's patent claims the use of complex winding patterns to enhance electrical efficiency for flux compression devices.
The ability to generate high levels of electrical energy from an FCG as related to the present invention allows for a safe and reliable means to initiate insensitive materials not only at a given point of the insensitive reactive material but also at multiple points within these materials. Thus, the present invention allows for multipoint initiation enabling tailorable effects in terms of munition fragmentation or blast output. Depending upon the target to be attacked, multipoint initiation can produce varying effects, i.e., enhance overall effectiveness, diminish munition output to provide control over collateral damage, or provide directed lethal output in given directions about the munition upon detonation.
While means exist to initiate explosive materials and to alter effects using multiple initiation sites within the explosives, no safe and reliable means is known when insensitive explosives are used as the main fill for munitions. Mainly, large booster systems or energy consuming electrical devices are required to assure initiation in these cases. The large booster systems in themselves compromise safety, whereas the electrical initiation approach requires undesired large electrical storage systems. Applications to weaponry have not been forthcoming because of these reasons. Use of an FCG as an energy source to power single point or multipoint initiation into detonation of insensitive materials has not been investigated previously.
There exists a need to initiate insensitive explosives in a safe and reliable manner, together with the ability to organize the initiation so that tailorable effects of munitions and weapons can be obtained. Such devices can integrate the FCG, loads, power supply, and initiators or detonators into a compact, autonomous package. With such device, sufficient energy is generated to initiate and sustain detonation in insensitive explosives and reactive materials. Prior state of the art initiation technology cannot accomplish this task in compact form. Essentially, the large boosters and/or large capacitors required prohibit their use as efficient, self-contained initiation systems. Further, multipoint initiation for tailorable effects cannot be accomplished since each initiation site would have to contain a sensitive explosive train together with a large sensitive booster, or the main power supply capacitor needed for slapper systems is far too large to be carried within the munition.
Although, conventional shaped charges and/or slapper detonators could possibly be used to initiate insensitive explosives, the energy sources become excessively large for either single point or multipoint initiation schemes. Further, safe and arming each and every initiator becomes a daunting task, which again involves inordinate amounts of space, volume, and weight. In addition, when a conventional explosive driven initiator is being used, there is no convenient means to avoid a catastrophic detonation of the main insensitive fill should the explosive within the initiator be accidentally detonated.